The present invention relates to a differential pressure transmitter and, more particularly, to an improved differential pressure transmitter that has a simplified mechanical structure, is more reliable and sturdy, and has low manufacturing and installation costs.
As known, pressure transmitters are widely used in industrial process control systems to sense one or more physical variables of a process fluid (pressure, flow, flow-rate, et cetera) by virtue of one or more differential measurements. Furthermore, a pressure transmitter allows to send to a remote monitoring device data/information related to the sensed physical variable.
Typically, a differential pressure transmitter is constituted by a main body that comprises an internal enclosure that includes a measurement chamber that accommodates a pressure transducer. A pressure sensor is usually used as a transducer, since it is well-known that it is possible to obtain easily, from one or more differential pressure measurements on the process fluid, by considering the appropriate boundary conditions, measurement values that relate also to other physical variables of interest (for example level, flow, turbulence and the like) that it would be more difficult to transduce directly.
In addition to said measurement chamber, the internal enclosure of the pressure transmitter accommodates primary electronic circuits, generally used to process the electronic signals that arrive from the pressure sensor. Generally, the main body of a conventional pressure transmitter also comprises an external enclosure that has protective and insulating functions. The external enclosure, in addition to fully enclosing the internal enclosure described above, accommodates some secondary electronic circuits meant to process the electronic signals that arrive from said primary electronic circuits and to handle communications with remote monitoring devices.
To perform the required sensing and measurement operations, the pressure transmitter must be placed in contact with the process fluid. For this purpose, a conventional differential pressure transmitter comprises one or more flanges that are coupled mechanically (by virtue of a screw/bolt system) to one or more manifolds that are suitable to convey the process fluid. Each flange is connected, by virtue of a further screw/bolt system, to the main body of the pressure transmitter, at at least one coupling surface, proximate to which a separation membrane is arranged. Typically, the separation membrane is arranged so as to have an outer wall that is exposed to the process fluid and an inner wall that is coupled hydraulically to the pressure sensor enclosed in the measurement chamber.
Known kinds of differential pressure transmitter have some drawbacks.
A first drawback arises from the fact that known transmitters have a relatively complicated mechanical structure that comprises, as mentioned, a plurality of containment enclosures that are mutually connected and connection flanges that are connected to the body of the transmitter by virtue of screws and bolts. A mechanical structure of this kind requires a relatively large number of operations for assembling the transmitter and for its installation in situ. This fact necessarily entails relatively high manufacturing and installation costs.
A second drawback that is certainly not secondary arises from the use of a system of screws and bolts to connect each flange to the main body of the transmitter.
Practice has in fact shown that if the bolts are not tightened very uniformly and with particular accuracy, mechanical stresses can arise at the separation membrane, these stresses being commonly called xe2x80x9cedge effectsxe2x80x9d. The intensity of these mechanical stresses is generally difficult to predict during design, since it depends on a plurality of factors, including asymmetries in the tightening of the connection bolts, uneven mechanical tolerances, ambient temperature variations, et cetera.
Experience has shown that so-called xe2x80x9cedge effectsxe2x80x9d on the separation membrane can cause measurement errors on the part of the pressure transmitter, since pressure variations may occur between the separation membrane and the pressure sensor. Often the extent of these measurement errors is not at all negligible, in view of the high performance that is generally required of a transmitter, especially in terms of accuracy and stability in the long term. Therefore, in order to contain these measurement errors, the operations for assembling the transmitter are relatively complicated and laborious: this inevitably entails an increase in manufacturing times and costs. Moreover, when these measurement errors occur in a transmitter that has already been installed, it is often necessary to resort to extraordinary maintenance interventions, which are particularly expensive in terms of both time and cost.
The main aim of the present invention is to provide a differential pressure transmitter that allows to solve the drawbacks described for known types of transmitter. Within this aim an aspect of the present invention is to provide a differential pressure transmitter that has a simplified mechanical structure requiring a relatively small number of operations for assembling and installing the transmitter. Another aspect of the present invention is to provide a differential pressure transmitter that has a mechanical structure that allows to limit to negligible levels the onset of edge effects on the separation membranes.
Thus, the present invention provides a differential pressure transmitter for sensing a physical variable related to a process fluid of an industrial system, which comprises:
a main body, which comprises a containment enclosure and a base that is associated with said containment enclosure; and
a secondary body, which comprises a first connection element, said first connection element comprising a first coupling surface at which said secondary body is associated with the base of said main body;
wherein said secondary body comprises at least one second connection element that comprises a second coupling surface at which said second connection element is associated with said first connection element, said second connection element being rigidly connected to said first connection element so as to constitute a single integrated mechanical structure together with said first connection element.
For a better understanding of the present invention, reference is made to the accompanying drawings and to the detailed description hereinafter, in which preferred but non-limitative embodiments of the differential pressure transmitter according to the present invention are illustrated.